Car juddering on acceleration

JOLOUJO

2001 FORD 500

2WD

MANUAL

180,000 MILES

I have a 2001 Mondeo which judders on fast acceleration-it seems to be fine if you accelerate very slowly. This started about week ago and gradually getting worse. It is petrol with an LPG conversion(fitted 2 years ago by previous owner). It has been into the garage for diagnostics but came back fine. We do not use the LPG, my only concern is to get the car running smoothly on petrol.

The only thing garage could suggest was change the spark plugs, which we have done but this was no improvement. It seems worse now not better. There is a new coil pack on it already.

I have been told it could be fuel starvation. Sorry to sound clueless, but how would we fix that?